It has previously been proposed to coat valves, and specifically valve elements, which are used to control gas flow in internal combustion engines by dipping a valve element into a melt of a material which is resistant to corrosion under high-temperature conditions (see German Patent DE-PS No. 944,642).
This process has the disadvantage that, after dipping the valve element into the melt, a comparatively long time is required for the heat diffusion treatment so that the coating material will adhere properly to the valve element. Additionally, the thickness of the coating thus obtained is essentially uniform throughout the treated area.
Valves which are highly stressed in use require differential thicknesses of armor-coating material, so that in those regions where they are most subject to wear, they are more thickly coated. Thus, in the region of the valve seat, the valves should have a coating thickness of from between 2 to 3 mm; the remainder of the valve element may, however, have a coating thickness in the order of several .mu. m. Such a thin coating may be entirely sufficient for the remainder of the valve element.
Coating materials which, for example, are nickel-base materials can also be applied in form of an armor-coating layer by Plasmarc or Argonarc methods; such coating materials will have high wear resistance and will be extremely hard even at elevated temperatures of over 773 K (500.degree. C.).
As the sizes of valves increase, difficulties arise with Plasmarc or Argonarc coating methods since the high temperatures with which the armor-coating material must be applied impinge the liquid coating material on a colder valve element, which leads to high internal stresses in the resulting coating. These internal stresses, due to the difference in temperature between the coating material in liquid phase and the colder valve element cannot be entirely removed by annealing, even heating to incandescence, to eliminate internal stresses, due to the high heat expansion limits and differences in temperature characteristics of the materials used for the valve element and the coating itself. Thus, upon additional stresses in use, or in further machining or manufacture, the armor-coating may be subjected to fissures, cracks, or other imperfections. The application process is somewhat similar to a welding process; using this application process in a semi-automatic application apparatus cannot exclude faults in the application or welding zones. Non-destructive quality control, even if very meticulous and careful, cannot detect defects within the coating, and thus defectively coated valve elements can be built into engines and used therein. In use, coating material may then break off, or be removed locally or entirely from the valve with consequential damage to the engine with which the valve element is used.